1. Field of the Invention
The present invention relates to a method and an apparatus for reading an image and, more particularly, to a method and an apparatus for reading an image in which an ununiformity and a variation in signal of each bit which is read by a photoelectric converting device are compensated and thereby enabling a stability of an output signal to be improved.
A method and an apparatus for reading an image according to the present invention are applied to an original reading section such as, for example, a facsimile apparatus or the like and, particularly, to a contact line sensor.
2. Description of the Prior Art
Hitherto, in image signal reading apparatuses such as a facsimile apparatus or the like, a photo sensor of one-dimensional photo diode type consisting of crystal silicon is used. In such a kind of photo sensor, there are drawbacks such that there is a limitation of a length and a yield of products is low in terms of a size and a working accuracy of a silicon monocrystal which can be manufactured. Therefore, in the case where a width of an original to be read is large (for example, 210 mm), it is necessary to reduce and form an original image on a photo sensor using a lens system and then read it.
However, the use of such a reducing lens system makes it difficult to miniaturize the photo sensing section because the optical length becomes long. On one hand, if each image area of a photo sensor is made small to maintain a high resolution, a large quantity of light is needed to obtain a sufficient signal current.
Consequently, the foregoing photo sensor is used in a reading apparatus of the low-speed type in which the reading time is made long or of the type in which a high resolution is not required.
On one hand, a photo sensor of the photoconductive type using amorphous silicon (hereinbelow, referred to as a-Si) has recently been proposed.
This photoconductive type photo sensor is manufactured by forming a thin a-Si film on the surface of a glass substrate by way of a vacuum deposition method; therefore, a photo sensor of a large area or of a long scale can be easily manufactured. Thus, even in the case where a width of an original is large as well, the original image can be read at an equal magnification and the reading apparatus can be easily miniaturized.
However, the photoconductive type photo sensor using a-Si has such a nature that the sensitivity characteristic varies in dependence on the radiation time of the light. For example, assuming that a photo current of the photo sensor when the light radiation time is zero is 100%, in the case where the light of 100 lux is radiated for about 1000 hours, the photo current decreases to a level of about 50%. When the photo current decreases to a level below 50%, it cannot effectively function as a photo sensor.
Further, the photoconductive type photo sensor using a-Si has such a nature that a photo current changes in dependence on a change in circumstances. In particular, a sensitivity of the sensor varies depending on a temperature change and the reading operation becomes unstable.
In addition, in case of using such a photo sensor in an image reading apparatus, there occurs a problem as well such that it becomes difficult to output an electric signal which accurately corresponds to an original image due to causes such as ununiformity of the amounts of light emission of a light emitting device array such as LEDs or the like, a difference between the light amounts in the central and peripheral portions of the array in case of using the lens array of the refractive index distribution type, ununiformity of the sensitivities of the photoelectric converting device array, or the like.
To solve the above-mentioned problems, the following methods have conventionally been adopted. One of them is a method whereby prior to reading an actual original image, a white original serving as a reference which has been prepared is first read and the actual read image signal is compensated by way of the reference value.
However, this method has a problem such that it is necessary to exchange the original whenever the original is read and the whole image processing time becomes long.
On one hand, there has been proposed an apparatus in which the optical system is rotated by only a predetermined angle to form an optical path between a photo sensor and a white portion provided in close proximity to the original portion to be read. However, this apparatus has a problem such that its structure is complicated and the adjustment is difficult because it has a movable section.
Further, there has also been proposed a method whereby' the pure white portion is read; the compensation signal to which each ununiformity mentioned above was reflected is stored; the actual read signal of the original image is compensated using this compensation signal; and the image information which accurately corresponds to the original image is eventually outputted. However, even such a method also has problems such that it is necessary to always provide the pure white portion in an original or to read the pure white original just before the original is read and therefore the original to be read is limited, and also this causes an arrangement of the reading apparatus to become complicated, and the like.